1. Field of the Invention
The present invention relates to a process for producing retinal which is a basic starting material for producing carotenoids that is important in the fields of medicines, feed additives and food additives and it also relates to intermediates for producing retinal.
2. Description of Related Art
Retinal is an important basic starting material for producing carotenoids, e.g., .beta.-carotene. As a method for the production of retinal, a method has been known, in which retinal is produced by oxidizing retinol (e.g., J. Chem. Soc. 411 (1944), JP63-233943A, Helv. Chim. Acta 40, 265 (1957) and JP7-103095B). However, this method has a problem in that it has required handling of retinol, which is sensitive to heat, light and oxygen. In addition to the above methods, the following methods are known, a method in which a carbon-increment reaction is conducted in the side chain of .beta.-ionone which is a C13 compound (e.g., Bull. Soc. Chim. Fr. 132, 696 (1995)) and a method in which a carbon-increment reaction is conducted in a side chain of cyclocitral which is a C10 compound (Chem. Lett. 1201 (1975)). These methods, however, require, as a starting material, .beta.-ionone or cyclocitral which is expensive on the market and produced by multistage processes. Hence, these methods are not always industrially satisfactory.